| Abstract |
Geothermal brine is saturated with silica (with respect to quartz) in the reservoir, can become supersaturated (with respect to amorphous silica) when steam is extracted or the brine cooled in a powerplant. The excess silica precipitates either as colloidal particles which may subsequently deposit on surfaces (pipe or rock fissure walls), or a vitreous film on those surfaces formed from monomeric silica. This scale may obstruct pipes and reinjection wells and limit the efficacy of heat exchangers. The transport of colloidal silica to the wall is influenced by temperature, particle size, flow velocity and also by pH, ionic strength and the species present at the wall, which affect the magnitude of the repulsive electrical double-layer force and the attractive, short-range London forces. The theory of colloid growth and transport is reviewed and timescales for key steps is calculated, for conditions representative of NZ geothermal plant. Data from a recirculating flow rig is presented, showing the effect of temperature, particle size, flow structure and surface morphology on the rate of deposition in mild steel pipes. Colloidal silica tends to deposit first in isolated clusters, whose size and location is influenced by chemical and local hydrodynamic conditions. |